When drilling and completing a well in an underground formation, a fluid or fluid-like substance having a density greater than water is typically used, e.g., a heavy weight drilling mud and water mixture. The dense mixture produces overbalanced hydrostatic pressures (i.e., pressures in excess of the formation pore pressures) in the well, e.g., to help prevent wellbore wall caving, to consolidate loose formations, or to control well pressure by minimizing the risk of excessive gas from the formation entering the wellbore.
However, the dense mixture tends to intrude into permeable portions of the formation, such as a productive interval. This intrusion can damage the productive interval, e.g., penetration of a water-based drilling fluid into a clay-containing formation can cause swelling and a loss of permeability. Damage to a productive interval may only be shallow (e.g., "skin" damage) and relatively easy to correct, but the damage may also be more extensive and permanent.
In a conventional well completion that includes a gravel pack (e.g., for sand control of a productive formation), a viscous as well as dense fluid (such as brine) may be used to entrain gravel particles and carry the stabilizing particles as a slurry into the face of the sandy formation to form the gravel pack. But the entraining fluid may cause further damage to the formation. Fluid loss control measures may also be required during a conventional gravel packing process, e.g., adding LCM "pills" or other fluid additives to control lost circulation when using a work string and backflushing tools to remove excess sand or gravel slurry. Coiled tubing and associated tools may also have to be run and nitrogen injected through the coiled tubing to bring a conventionally gravel packed well into production, adding still more risk of formation or other damage.
Significant costs are typically required for a drilling rig or other well intervention unit to be on-site during a conventional gravel packing process. The rig is typically used periodically throughout the conventional gravel packing process, e.g., to place, support, reposition, activate, and/or remove gravel packing tools downhole. The rig may be required to be on-site for many days during a conventional gravel packing process.
Use of a rig allows one or more packers attached to a work string to isolate a productive interval or zone during gravel packing. The isolated zone and work string allow a pressurized, but less dense fluid to be used to entrain the sand or gravel without exposing other portions of the wellbore to the pressurized fluid. But backflushing steps and means for removing excess slurry are typically required when a packer is used. In addition, placing, backflushing, and removing packers and other tools add costly rig time and entail other damage risks.